Membrane and Desalination Technologies - TCE Moodle Website

482 P. Kajitvichyanukul et al.
wells exceeds the MCL for NO 3 – and long-term trends indicate increasing concentrations of
NO3 – in groundwater (21).
NO 3 – are water-soluble and do not bind to soil thus causing their migration to groundwater
sources (22). Once consumed, NO3 – are converted into nitrites (NO2 ) in the body. Methemoglobinemia
or blue baby syndrome in infants is related to the ingestion of water with high
NO3 – concentration (23). Moreover, there exists an increasing concern that consumption of
water containing high NO3 – may lead to some other diseases including carcinoma, malformation,
and mutation when transformed into nitrosoamines (24–26).
Removing NO3 – (NO3 ) and NO2 from drinking water has gained tremendous attention
in recent years. It is reported by the World Health Organization that NO3 – concentration in
drinking water should be lower than 10 mg-N/L (27). In the USA, US EPA set MCL of 10 mg
NO 3 – -N/L and 1 mg nitrite-N/L. The European standard for NO2 – is stricter (0.03 mg-N/L) to
account for the direct toxic effects from NO2 (28). A recommended level for NO3 – is 5.6
mg-N/L for the European Community (29). However, these standards are difficult to meet by
merely controlling the contamination at the source; denitrification processes constitute the
major technology for the removal of NO3 – from potable water.
2.5.1. Biological Nitrate Removal
Biological nitrate removal is conducted by a wide variety of organisms by either assimilatory
or dissimilatory pathways (30, 31). In assimilatory pathway, organisms use nitrate, rather
than ammonia, as a biosynthetic nitrogen source for nitrate reduction. This process can occur
in either aerobic or anaerobic condition. Assimilatory nitrate reduction occurs in the absence
of more reduced inorganic nitrogen species (e.g., ammonia). In this process, the inorganic
nitrogen is converted to organic nitrogen, thus, no net removal of inorganic nitrogen is
accomplished.
Dissimilatory pathway consists of two groups of prokaryotic organisms. In the first
pathway, dissimilatory nitrate reduction to ammonia, NO 2 – is reduced to either nitrite or
ammonia by anaerobic and facultative anaerobic bacteria. In denitrification pathway, nitrate
is reduced to NO 2 – and finally to gaseous nitrogen (N2) as the end products by the second
group of bacteria, facultative anaerobic bacteria.
2.5.2. Biological Denitrification
Biological denitrification is generally used in treating municipal and industrial wastewaters.
Recently, research has lead to more knowledge and experience in using biological
denitrification as a viable technology in nitrate removal from drinking water. Denitrification
is a biological process that reduces NO3
and finally to N2 as shown in Eq. (1) (31):
to NO2 , nitric oxide (NO), nitrous oxide (N2O),
NO3 ! NO2 ! NO ! N2O ! N2: (1Þ
Denitrification takes place under anoxic conditions. Oxygen concentrations in the range of
0.1–0.2 mg/L can exert inhibitory effects on these processes (32). Both heterotrophic and
autotrophic bacteria can be involved in denitrification (29). Heterotrophic bacteria under